package DailyPractice.Day34;
import java.math.BigInteger;
import java.util.*;
/**
 * 养兔子
 * @author lumin
 * @date 2022/06/01 22:03
 **/
public class Num2{
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        while(sc.hasNext()){
            int n=sc.nextInt();
            BigInteger oldRabbit=new BigInteger("1");
            BigInteger newRabbit=new BigInteger("0");
            for(int i=0;i<n-1;i++){
                BigInteger temp=newRabbit;
                newRabbit=oldRabbit;
                oldRabbit=oldRabbit.add(temp);
            }
            System.out.println(oldRabbit.add(newRabbit));
        }
    }
}
